The development of bitumen-in-water emulsions has provided renewed interest in burning vast stores of viscous hydrocarbons, variously called bitumen, native bitumen, tar, natural asphalt, viscous crude oil and heavy crude. See in this regard the paper Evaluation of Handling & Combustion Characteristics of A Bitumen-in-Water Emulsified Fuel in a Commercial Utility Boiler presented in December 1989 by Blair A. Kennedy at the Power-Gen '89 Conference. See also, U.S. Pat. No. 4,618,348 to M. E. Hayes et al. One commercially available bitumen-in-water emulsion product, known as Orimulsion, is made with about 70% Venezuelan Orinoco crude and about 30% water. Reserves of Orinoco crude are estimated at 1.2 trillion barrels, but it typically has high sulfur and vanadium contents which deter widespread commercialization.
The removal of sulfur, though desirable to limit the emissions of SO.sub.x compounds to the atmosphere during combustion, has to date been so impractical in terms of complexity of proposed reactors and reagents that none have been applied to bitumen. The oxides of sulfur are, however, known to contribute to the formation of acid rain and their removal must be taken into account in any economic study. Removal of contaminants prior to combustion would be particularly advantageous because these contaminants are more concentrated prior to their reaction with oxygen to form SO.sub.x compounds during combustion. The removal of SO.sub.x compounds by post-combustion methods involves the treatment of higher molecular weight compounds entrained in high-temperature, high-volume flue gases and requires that substantial capital and operating costs be applied to each major source of combustion. Yet, no practical method is available for precombustion removal of organosulfur compounds from bitumens, and the art has focused on post-combustion techniques which themselves cause difficulties in terms of solid disposal.
Pre-combustion removal of contaminants would have the advantage of being conducted on the preoxidized state of the contaminant. Pre-combustion techniques could be conducted at any point in the fuel handling and distribution stream prior to combustion. Pre-combustion treatment methods proposed for coal and light petroleum fractions have involved physical, chemical and biological treatments. Unfortunately, much of the offending sulfur compounds cannot be removed physically, and chemical treatment requires large investments in capital and high operating costs due to the use of hydrogen, solvents or the like.
For petroleum oils and coals, prior workers have attempted to remove both pyritic and organic sulfur through the use of microbial techniques. For a recent review of this, see Monticello and Finnerty, "Microbial Desulfurization of Fossil Fuels", Ann. Rev. Microbiol. 1985 (39:371-89). The discussion is primarily directed to the underlying biochemistry, not to the practical implementation of it, especially for bitumens which pose unique handling and processing problems. See also, Lee and Yen, "Sulfur Removal from Coal Through Multiphase Media Containing Biocatalysts", J. Chem. Tech. Biotechnol., 1990, vol.48, pp.71-79.
Several early patents have described large-scale processes for removing sulfur from low-viscosity oils. For example, in U.S. Pat. No. 2,521,761, Strawinski discloses using any of a variety of known microorganisms, but employs a "diverter" as carbon source as an alternative to the petroleum. Unfortunately, the diverter such as sugar or starch is costly. Later, in U.S. Pat. No. 2,574,070, Strawinski disclosed first converting sulfur to sulfates and then removing the sulfates in a second stage microbiological reaction by converting them to hydrogen sulfide. In U.S. Pat. No. 2,641,564, Zobell also teaches using a microbiological catalyst to remove sulfur from petroleum by reaction with hydrogen to produce hydrogen sulfide. These reactions are necessarily conducted on liquid petroleum and require the use of complex gas-liquid contact devices and produce large quantities of foul-smelling and dangerous by-products.
Other processes have also failed to be implemented because they did not offer the promise of economical operation. In U.S. Pat. No. 2,975,103 Kirshenbaum teaches contacting liquid petroleum with aerobic bacteria in a liquid-liquid contact column to oxidize the sulfur to sulfates and then removing the sulfates by precipitation with CaO, BaO or the hydroxides. This process, while requiring sophisticated equipment for operating on liquid oils could not be expected to operate on bitumen which is not liquid at temperatures effective for most biochemical reactions.
Similarly, in U.S. Pat. No. 4,861,723, Madgavkar treats coal with microorganisms and follows the reaction with a required step of removing the aqueous phase prior to combustion.
It would be desirable to provide a process capable of removing sulfur from bitumen which could be implemented without complex and costly equipment or the use of expensive or hazardous chemicals. It would be especially desirable if these criteria could be met to produce a fuel which could be burned without any requirement that the aqueous phase be removed prior to combustion.